[BUG] Have a public ECR repo from common images generated in walkthrough

[suniltheta]

Describe the bug
Today for most of the walkthrough we need to generate the app or other images locally to test them out. Images such as grpc_cli takes longer time to generate and is frustrating.

As noted in the closed PR it took 10+ minutes to get the created #476

To improve the experience of users running these walkthrough we can give option to pull them simply from a public ECR repos.

Platform
EKS, ECS, EC2, etc.

To Reproduce
Try step 2 in this walkthroughs/howto-grpc-ingress-gateway/README.md

Expected behavior
Option to just pull the image from public ECR.